Microwave moisture measuring apparatus

ABSTRACT

Microwave moisture measuring apparatus wherein the output of a variable frequency microwave source is beamed by lens-corrected horns through a moisture containing material and then delayed in time and finally heterodyned with the original output of the microwave source to produce a predetermined beat frequency which can be amplified and then linearly detected.

MICROWAVE MOISTURE MEASURING APPARATUS George R. Mounce, Willowdale,Ontario, Canada Assignee: Electronic Associates of Canada,

Ltd., Downsview, Ontario, Canada Filed: Dec. 18, 1973 Appl. No.: 425,937

Inventor:

US. Cl 324/585 A int. Cl G0lr 27/04 Field of Search 324/58.5 A, 58 A, 57H References Cited UNITED STATES PATENTS 8/1966 Heald 324/585 A Nov. 26,1974 3,50l,692 3/l970 Kluck 324/585 A 3,534,260 10/1970 Walker 3,693,0799/1972 Walker 324/585 A Primary Examiner-Stanley T. KrawczewiczAttorney, Agent, or Firm-Shenier & OConnor [57] ABSTRACT Microwavemoisture measuring apparatus wherein the output of a variable frequencymicrowave source is beamed by lens-corrected horns through a moisturecontaining material and then delayed in time and tinally heterodynedwith the original output of the microwave source to produce apredetermined beat frequency which can be amplified and then linearlydetected.

28 Claims, 9 Drawing Figures 22-Z3GH IOOH BACKGROUND OF THE INVENTION Inmicrowave systems for measuring moisture in sheet materials such aspaper and the like, free water absorbs microwave energy due to molecularresonance. The greatest attenuation of microwave energy occurs when theexciting frequency corresponds to the natural resonant frequency of themolecules. One such absorption peak occurs in the band from to GHz. Onesystem of the prior art employs a microwave source operating in thisfrequency band which is coupled to a first horn antenna on one side ofthe paper web. A second horn antenna on the other side of the paper webis aligned with the first antenna and receives energy radiated from thefirst antenna. The output of the receiving antenna is applied to acrystal detector. Since the energy radiated from the first antennapasses through the paper web, it will be attenuated as a function of themoisture content of the web. The transmitting and receiving horns areusually directed at right angles to the web.

Such system suffers three defects. The first defect is that it isimpossible to prevent some microwave energy from being reflected back tothe transmitting antenna either from the receiving antenna or from thepaper web. Such reflections will change the apparent power radiated fromthe transmitting antenna, depending upon the phase of the reflectedsignals which will vary with any change in the spacing between the twoantennas, with any variations in the position of the paper web relativeto the two antennas, and with any change in frequency. Since it isimpossible to maintain an absolutely constant spacing between all partsof the system, the apparent power radiated from the transmitting antennawill vary because of the changing phase angle of the reflections. Thischanges the output from the crystal detector even though the moisturecontent of the paper web may be constant.

The second defect is that a conventional transmitting horn antennaproduces a divergent beam having a convex wavefront, while aconventional receiving horn antenna responds best to a convergent beamhaving a concave wavefront. This inherent mismatch in the curvature ofthe two wavefronts greatly increases the amplitude of reflectionsbetween the two horns.

The third defect is that crystal detectors used as straight rectifiershave a very limited sensitivity, since they are operated in a square-lawregion because of the relatively low power output of the microwavesource. For the levels of moisture commonly encountered in wet or damppaper webs, the attenuation is so great that the output of the detectoris extremely small. Moreover, operation of detectors in the square-lawregion at low power levels results in a high incremental impedance,producing correspondingly large noise voltages which further limit theminimum detectable signal.

In order to reduce the effect of reflections upon the output of thedetector, prior art systems have employed a frequency modulatedmicrowave source. The extend of frequency modulation is such as tochange the phase of reflections by 360 or any integral multiple thereof.The energy output from the receiving horn will thus pass through anintegral number of cycles of variation;

and if the variation is small, the average output of the detector willaccurately represent the moisture content of the damp paper web.However, in most cases the amplitude of reflections is not smallcompared with the average amplitude. Since the detector is operated in asquare-law region, its average output in the presence of strongreflections will always be greater than the average amplitude of signalscollected by the receiving horn. Accordingly, the simple frequencymodulation systems of the prior art produce excessive outputs from thedetector in the presence of strong reflections.

SUMMARY OF THE INVENTION My invention contemplates the provision of aheterodyne or beat freqnency system wherein the detector is subjectedboth to strong signals from the frequency modulated microwave source andto delayed and attenuated signals from the receivng horn antenna. Thestrong signal from the microwave source acts as a local oscillator onthe detector and forces it out of the highimpedance, high-noise,square-law region into a lowimpedance, low-noise, linear region. Thisprovides high sensitivity irrespective of the extent of attenuation inthe damp paper web. Furthermore the adverse effect of strong reflectionsis substantially eliminated, since linear detection insures that theaverage output of the detector will accurately represent the averageamplitude of signals at the receiving horn over an integral number ofcycles. My invention further contemplates the provision of dielectricconverging lenses for the transmitting and receiving horns so that thecompensated horns will produce and respond best to parallel beams havingplanar wavefronts. This greatly improves the match between the two homsand correspondingly reduces the amplitude of reflections.

One object of my invention is to provide microwave moisture measuringapparatus employing a heterodyne or beat frequency detector.

Another object of my invention is to provide microwave moisturemeasuring apparatus employing linear detection.

a further object of my invention is to provide microwave moisturemeasuring apparatus in which the adverse effects of reflections aresubstantially eliminated.

A still further object of my invention is to provide microwave moisturemeasuring apparatus in which mismatch reflections between horns aregreatly reduced by dielectric lenses.

Other and further objects of my invention will appear from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings whichform part of the instant specification and which are to be read inconjunction therewith and in which like reference numerals are used toindicate like parts in various views:

FIG. 1 is a schematic view showing a first embodiment of my inventionemploying two pairs of horn antennas. I

FIG. 2 is a graph showing the variation in frequency of the microwavesource as a function of time.

FIG. 2a is a graph showing the corresponding variation in the output ofthe detector as a function of time.

FIG. 3 is a graph on an enlarged scale showing variations in thedetector output due to both strong and weak reflections.

FIG. 4 is a schematic view illustrating a second embodiment of myinvention employing only a single pair of horn antennas.

FIG. 5 shows the design details of a converging dielectric lens havingtwo refracting surfaces.

FIG. 6 shows the design details of a converging dielectric lens having asingle refracting surface.

FIG. 7 is a front view of a stepped approximation to a double refractionlens which gives good results in practice.

FIG. 8 is a side view of the lens of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more particularlyto FIG. 1 of the drawings, my system may be used for measuring themoisture content of a horizontally disposed damp paper web 8. A 100 Hzoscillator 10 provides an output waveform of trapezoidal shape. The flattop and bottom of the trapezoid may each have a duration of 2.5milliseconds; and the rising and falling ramps may each have a durationof 2.5 msec. Oscillator 10 controls the frequency of a variablefrequency microwave source 12 such as a backward wave oscillator. Thefrequency of BWO 12 is varied by oscillator 10 between 22 GHz and 23 GHzwhich lies in the middle of the absorption band of the water molecule.Energy from oscillator 12 is coupled through a microwave isloator 14 tothe primary input port of a directional coupler 16. The primary outputport of directional coupler 16 is coupled to an upwardly directedtransmitting horn antenna 20 which is positioned below web 8. Positionedabove web 8 and aligned with the axis of horn 20 is a receiving hornantenna 22 which is coupled to a delay line 24. The output of delay line24 is coupled to a downwardly directed second transmitting horn antenna26 which is positioned above web 8. Positioned below web 8 and alignedwith horn 26 is a second receiving horn antenna 28. Horn 28 is coupledthrough a microwave isolator 30 to a secondary input port of directionalcoupler 16. The secondary output port of directional coupler 16 iscoupled to a linear diode detector 32. The output of detector 32 iscoupled to an amplifier 34 the output of which is applied to a band-passfilter 36. Filter 36 may have a sharp lower cutoff at 13 KHz and a sharpupper cutoff at 19 KHz. The output of filter 36 is coupled to a lineardiode detector 38, the output of which is applied to a low-pass filter40 which may have a sharp cutoff at 180 Hz. I-Iorns 20, 22, 26, and 28are mounted on a U-shaped member 9 which may be moved left and right toscan across the width of web 8. At the mouth of horn 20 is moutned aconverging dielectric lens 20a. Lens 20a may, for example, be of thesingle refraction type having a plane outer surface and a curved innersurface. Converging lenses 22a, 26a, and 280 are similarly mounted atthe mouths of respective horns 22, 26, and 28. The beams fromlens-corrected horns 20a and 26a are thus parallel with planarwavefronts.

In operation of the embodiment of FIG. 1, isolator 14 prevents reflectedenergy from being applied backwardly to oscillator 12 so that itsstability will not be impaired. Most of the energy applied at theprimary input port of directional coupler 16 passes to the primaryoutput port thereof; but a certain portion, de-

pending upon the coupling factor, appears at the secondary output portand is applied to heterodyne detector 32. The energy at the primaryoutput port of directional coupler 16 is radiated by horn 20 and passesthrough web 8 to receiving horn 22. The energy received by horn 22 isless than that transmitted by horn 20 because of attenuation orabsorption caused by the water molecules in web 8. The energy fromreceiving horn 22, after delay in line 24, is radiated from horn 26 andpasses downwardly through web 8 to receiving horn 28. The energyreceived by horn 28 is less than that transmitted by horn 26 because ofattenuation or absorption in web 8. The purpose of isolator 30 is toprevent passage of energy backwardly from coupler [6 to horn 28, whilepermitting energy from horn 28 to pass essentially unattenuated to thesecondary input port of directional coupler 16. Most of the energy atthe secondary input port of directional coupler 16 passes to thesecondary output port thereof and thence to heterodyne detector 32.

Detector 32 thus receives two signals. A first signal is that appearingat the primary input port of directional coupler 16. This first signalhas a large amplitude and may be considered the equivalent of a localoscillator signal. The second signal is that appearing at the secondaryinput port of directional coupler 16. this second signal is greatlyattenuated since it has passed through web 8 twice. This second signalis also delayed in that it has passed through coupler 16, horn 20, horn22, delay line 24, horn 26, horn 28 and isolator 30.

Assume that the overall time delay for the passage of the signal fromthe primary input port of directional coupler 16 to the secondary inputport of directional coupler 16 is 0.04 microsecond. During the flat topand bottom portions of the trapezoidal modulating waveform fromoscillator 10, the signals at the primary and secondary input ports ofdirectional coupler 16 will have the same frequency, that is, either 22GHz or 23 GHz. Hence during these periods the two signals applied todetector 32 will produce a difference or beat frequency of zero. Duringthe period when the output of oscillator 10 is rising and, for example,the frequency of oscillator 12 is increasing from 22 GHz to 23 GHz, thefrequency at the secondary input port of directional coupler 16 will beless than that at the primary input port thereof. Correspondingly duringthe period when the output of oscillator 10 is falling and the frequencyof oscillator 12 is decreasing from 23 GI-Iz to 22 GHZ, the frequency atthe secondary input port of directional coupler 16 will be greater thanthat at the primary input port thereof. It may be shown that thedifference frequency between the signals at the primary and secondaryinput ports of directional coupler 16 is equal to the product of therate of change of frequency of microwave oscillator 12 and the overalltime delay of .04 usec) for energy to pass from the primary input portof directional coupler 16 to the secondary input port thereof. For boththe positive and negative ramps of the trapezoidal output of oscillator10, the rate of change of frequency of oscillator 12 is 1 GHz per 2.5msec or 4( I0) I-Iz/sec. Accordingly the difference in frequencies ofsignals at the input ports of directional coupler 16 during the risingand falling ramps of the trapezoidal output of oscillator 10 is4(l0)(0.04)(l0) 16 KHz. This is the center frequency of band-pass filter36.

Referring now to FIG. 2, there is shown the 100 Hz waveform ofoscillator which controls the frequency modulation of oscillator 12.FIG. 2a shows the corresponding output from detector 38. It will benoted that the output from detector 38 comprises a 200 Hz square wave.During those periods when the output from oscillator 110 is constant,the difference frequency output from detector 32 is zero; and the outputfrom detector 38 is also zero. During those periods when the output ofoscillator 18 is changing, detector 32 provides a difference frequencyoutput of 16 KHz which lies in the pass band of filter 36; and detector38 may provide, for example, a positive output. It will be noted thatthe output from detector 38 is not a perfect square wave sincesults inshort duration rising and decaying transients in the output of detector38.

While oscillator 10 may provide a pure triangular waveform, a clippedtriangular or trapezoidal waveform with flat top and bottom portions ispreferred, since this produces extended periods of time for which theoutput of detector 32 is zero. This permits energy stored in thereactive elements of filter 36 to completely dissipate so that eachburst of 16 KHz energy from heterodyne detector 32 can be treated as aseparate entity by band-pass filter 36. If the output of oscillator 10were a pure and unclipped triangular waveform with no flat dwellportions during which the energy stored in filter 36 could decay, thenthe output of detector 38 would be dependent upon the phase shiftbetween successive 16 KHz bursts from detector 32. If there were nophase shift between successive bursts from detector 32, then the outputfrom detector 38 would be a constant direct-current. If, however,successive 16 KHz bursts from detector 32 had any relative phase shiftother than 0, then the output of detector 38 would momentarily decreaseand then rise again to its original value. The worst case would occurwhen the phase shift between successive KHz bursts from detector 32 is180. In this case the output of detector 38 would momentarily drop tozero and then rise again to its original value.

It is desired that low-pass filter 40 integrate over one full cycle ofthe 200 Hz square wave output from detector 38. Accordingly filter 40should have a cutoff frequency less than 200 Hz, as for example 180 Hz,in order to eliminate 200 Hz and higher harmonic ripple in thedirect-current output.

Assume that the time delay for passage of signals from the throat ofhorn to the throat of horn 22 is 0.002 usec. For any mismatch betweenhorns 20 and 22, this same amount of time will be required forreflections from the throat of horn 22 to be returned to the throat ofhorn 20. The total time delay for reflections back and forth between thethroats of horns 28 and 22 will be 0.004 usec. Similarly the total timedelay for reflections back and forth between the throats of horns 26 and28 will also be 0.004 usec. During those periods when the frequency ofoscillator 12 is changing, horns 22 and 28 receive two signals ofdifferent frequencies, due to horn mismatch reflections, which arecoupled to the secondary input port of directional coupler 16. Thefrequency difference between these two signals is equal to the productof the rate of change of frequency of microwave oscillator 12 and thetotal time delay for reflected signals to pass back and forth betweenthe throats of a pair of opposed horn antennas. Thus the frequencydifference between the two signals applied to the secondary input portof directional coupler 16 is 4(10)(0.004) (l0) 1,600Hz.

As may be seen by reference to F IG. 3, during the period when thefrequency of oscillator 12 is changing, the output of detector 38 willexhibit a 1,600 l-Iz variation about its average pulse amplitude. Duringeach 2.5 msec period when the frequency of oscillator 12 is changing,there will occur 2.5( 10)'( 1,600) 4 cycles of 1,600 Hz variation due toreflections. It is desired that there be an integral number of cycles ofvariation so that the total area under each pulse will be constantirrespective of the magnitude of reflections and hence of the amplitudeof the 1,600 Hz variation. When dielectric lenses are used, the 1,600 Hzvariation is substantially sinusoidal as shown greatly exaggerated inscale by curve 38 of FIG. 3. However, when the dielectric lenses areremoved, there results a 1,600 Hz variation of substantially sawtoothwaveform as shown approximately to scale by curve 38a of FIG. 3. Thevariation in waveform 38a may be expressed approximately sin6+%sin20+/asin 30,

where 0 27r( 1,600) r radians. Waveform 38a includes not only a 1,600 Hzfundamental component but also appreciable 3,200 Hz and 4,800 I-Izsecond and third harmonic components. These harmonic components resultbecause the horn mismatch is so severe that two and three round-tripreflections between horns 20 and 22 still result in an appreciableamplitude of the reflected signal. The use of lens-corrected hornsgreatly reduces the amplitude of all mismatch reflections. Theattenuation of the second and third harmonic components is so great thatthey are substantially suppressed; and only the fundamental component isdiscernible.

The system of FIG. 1 requires appreciable space because of the use offour horn antennas. Furthermore it does not measure the moisture in asingle area but instead measures the average moisture in two distinctareas, which somewhat limits the discriminiation of the measurement. Forthese reasons it would be advantageous if only a single pair of hornantennas were required.

Referring now to FIG. 4 of the drawings, the primary output port of thedirectional coupler 16 is connected to the 0 port of a polarizationseparator 17, having a port and a common port. The common port ofpolarization separator 17 is coupled to a horn antenna 19 which ispositioned below web 8. Horn 19 is directed upwardly, but at anappreciable angle from the vertical. Positioned above web 8 and alignedwith the axis of horn I9 is a horn antenna 21 which is coupled to thecommon port of a polarization separator 23, having a 0 port and a 90port. The output from the 0 port of polarization separator 23 is coupledto delay line 24. The output of delay line 24 is applied to a section ofwave guide 25 which is twisted through 90 thereby to rotate the plane ofpolarization through 90. The output from the 90 twist section'25 isapplied as an input to the 90 port of polarization separator 23. Theoutput from the 90 port of polarization separator 17 is coupled to asection of wave guide 27 which is twisted through 90 thereby to rotatethe plane of polarization through 90. The output from 90 twist section27 is coupled through microwave isolator 30 to the secondary input portof directional coupler 16. I-Iorns 19 and 21 are mounted on a U-shapedmember 9 which may be moved left and right to scan across the width ofweb 8. At the mouth of horn 19 is mounted a dielectric lens 19a; and atthe mouth of horn 27 is mounted a lens 210. Lenses 19a and 210 are ofthe double-refraction type having a curved outer surface and a planarinner surface.

In operation of the embodiment of FIG. 4, microwave enegy from source 12passes through isolator 14, to the primary input port of directionalcoupler 16. Most of this energy passes to the primary output port ofcoupler l6 and thence through polarization separator 17 to horn 19. Thisenergy is radiated from horn l9 and passes through web 8 at anappreciable angle from the normal. For this forward transmission path,horn 19 transmits; horn 21 receives; and the plane of polarization isThe energy received by horn 21 passes through polarization separator 23and delay line 24 to 90 twist section 25. The energy from twist section25 passes through polarization separator 23 to horn 21. The energy isradiated from horn 21 and passes through web 8 to horn 19. For thisreturn transmission path, horn 21 transmits; horn 19 receives; and theplane of polarization is 90. The energy received by horn 19 passesthrough polarization separator 17 to 90 twist section 27. Section 27rotates the plane of polarization back to 0 and the energy passesthrough isolator 30 to the secondary input port of directional coupler16. The 90 twist section 27 is provided so that both the primary andsecondary input ports of coupler 16 will receive energy with the sameplane of polariazation. This insures that the summation of signals atthe secondary output port of coupler 16 is governed solely by itscoupling factor and is independent of the orientation of detector 32within the secondary output port of coupler 16.

Horns 19 and 21 as well as the short sections of wave guide joining horn19 to the common port of separator 17 and joining horn 21 to the commonport of separator 23 should have symmetry about the X and Y axes inorder to pass with equal facility energy having a polarization of either0 or 90. Accordingly, horns l9 and 21 may have, for example, either asquare or a circular cross section.

In FIG. 4, the 0 polarized radiation from horn l9 passes upwardlythrough web 8 at an angle of approximately 45 from the normal; and 90polarized radiation from horn 21 passes downwardly through web 8 at thesame angle from the normal. This effectively increases the apparentthickness of web 8 by approximately 41 percent and results in a greaterattenuation of microwave energy appearing at the secondary input port ofcoupler 16. Furthermore any 0 polarized energy from horn 19 which isreflected from web 8 will be directed downwardly and to the left andwill not be returned directly to horn 19. Similarly any 90 polarizedenergy from horn 21 which is reflected from web 8 will be directedupwardly and to the right and will not be returned directly to horn 21.This greatly reduces the coupling of web reflections back to either ofhorns 19 and 21 and consequently reduces the adverse residual effect ofvariations in the position of the web relative to the two horns. Becauseof the greatly increased power gain of lens-corrected horns, the size ofthe horns may be reduced by 40 percent while at the same time achievinga 4 db increase in overall gain. The reduction in horn size permits acorresponding reduction in horn separation where, as in FIG. 4, thebeams pass through the web at an appreciable angle from the normal.

- The 0 polarized energy from horn l9 and the polarized energy from horn21 both pass through web 8 in the same area. This increases thediscrimination of the microwave moisture measurement.

Referring now to FIG. 5, there is shown the design details for adouble-refraction lens. In the equations, f is the focal length of thelens which is substantially equal to the length of the horn, d themaximum thickness of the lens which should be an integral number of halfwavelengths, and n is the refractive index of the lens material which isequal to the square root of its relative dielectric contant.

FIG. 6 shows the design details for a single-refraction lens. The lensesof FIGS. 5 and 6 are of the planoconvex type. The lens of FIG. 5 has ashape factor of +1, while the lens of FIG. 6 has a shape-factor of -1.If the convex surfaces are limited simply to being spherical, then thelens of FIG. 5 is preferable since it is closest to the shape factor of+0.714 for minimum spherical aberation. However, optical precision isneither needed nor required. As a practical matter, either of the lensesof FIGS. 5 and 6 is more than adequate; and in fact even relativelycrude approximations to these lenses yield excellent results inpractice.

FIGS. 7 and 8 show a step-wise approximation to a piano-convex lenswhich gives satisfactory results in practice. Lens 41 comprises a 4.000inch square base section 42, a 3,455 inch diameter circular intermediatesection 43, and a 2.300 inch diameter circular top section 44. Each ofsections 42, 43, and 44 may have a thickness of 0.186 inch, so that themaximum lens thickness is 0.558 inch. The lens may be integrally formedof Teflon, which is a brand of tetrafluoroethylene polymer made by E I.du Pont de Nemours and Co. Lens 41 is designed for use with a hornhaving a square cross section with a four inch mouth and a length of 11inches.

It will be seen that I have accomplished the objects of my invention. Mymicrowave moisture measuring apparatus employs a heterodyne or beatfrequency detector which is subjected both to a strong signal from thefrequency modulated source and to a delayed signal which has beenattenuated by virtue of its having passed twice through the moisturecontaining web. The strong signal from the microwave source causes thebeat frequency detector to operate in a substantially linear manner.This increases the sensitivity of the detection of the attenuated anddelayed signals. The use of lens-corrected horns greatly reducesmismatch reflections and increases the power gain. The adverse effect ofresidual reflections is substantially eliminated, since linear detectioninsures that the average output of the detector represents the averageamplitude of signals over an integral number of cycles. The effect ofresidual reflections may be rendered negligible by narrowing the passband of filter 36 from 6 KI-Iz to ZKHz. Filter 36 should have a sharplower cutoff at 15 K112 and a sharp upper cutoff at 17 KHz. Thus filter36 can respond only to modulating frequencies up to 1 KHz or 1,000 I-Izwhich is less than the 1,6000 Hz fundamental frequency component ofmismatch reflections. The output of detector 38 is now as shown in FIG.2a with negligible ripple discernible during each positive pulse.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of myclaims. It will be further understood that various changes may be madein details without departing from the spirit of my invention. It istherefore to be understood that my invention is not to be limited to thespecific details shown and described.

Having thus described my invention, what I claim is:

l. Microwave moisture measuring apparatus including in combination avariable frequency source of microwave energy, control means forcyclically varying the frequency of the source, means including alenscorrected horn for radiating energy from the source through amoisture containing material, said material attenuating the energypassing therethrough as a function of its moisture content, a firstdetector, and means coupling both the source and the attenuated energyto the first detector.

2. Apparatus as in claim 1 further including a bandpass filter, a seconddetector, and means including the filter for coupling the first detectorto the second detector.

3. Apparatus as in claim 1 wherein the control means cyclically variesthe source frequency at a certain repetition frequency, the apparatusfurther including an amplifier, a band-pass filter, a second detector,means including the amplifier and the band-pass filter for coupling thefirst detector to the second detector, a lowpass filter, and meanscoupling the second detector to the low-pass filter.

41. Apparatus as in claim 3 wherein the low-pass filter has a cut-offfrequency less than twice said repetition frequency.

5. Apparatus as in claim 1 wherein the construction of the control meansis such that the source frequency cyclically varies in accordance with agenerally triangular waveform having equal positive and negative slopes.

6. Apparatus as in claim 1 wherein the construction of the control meansis such that the source frequency cyclically varies in accordance with atrapezoidal waveform having flat top and bottom portions of appreciableextent. I

7. Apparatus as in claim ll wherein the means coupling the attenuatedenergy to the first detector includes a delay line.

8. Apparatus as in claim 1 wherein the means coupling the source and theattenuated energy to the first detector includes a directional coupler.

9. Apparatus as in claim 1 wherein the radiating means includes amicrowave isolator.

10. Apparatus as in claim 11 wherein the means coupling the attenuatedenergy to the first detector includes a microwave isolator.

ll. Apparatus as in claim ll wherein the radiating means includes meansfor passing source energy twice through the material.

12. Apparatus as in claim ll wherein the radiating means includes afirst transmitting horn antenna disposed on one side of the material, afirst receiving horn antenna disposed on the other side of the materialand oriented to receive energy from the first transmitting horn, asecond transmitting horn antenna disposed on said other side of thematerial, means coupling the first receiving horn to the secondtransmitting horn, and a second receiving horn antenna disposed on saidone side of the material and oriented to receive energy from the secondtransmitting horn.

13. Apparatus as in claim 12 wherein the means coupling the firstreceiving horn to the second transmitting horn includes a delay line.

14. Apparatus as in claim 1 wherein the radiatingmeans includes a firstand a second horn antenna disposed on opposite sides of the material andoriented to transmit and receive energy from one another, a first and asecond polarization separator each having a common port and a pair ofpolarization ports, means coupling the source to one polarization portof the first separator, means coupling the common port of the firstseparator and the first horn, means coupling the common port of thesecond separator and the second horn, and means including a firstpolarization rotator for coupling one polarization port of the secondseparator and its other polarization port.

15. Apparatus as in claim 14 wherein the means coupling the twopolarization ports of the second separator includes a delay line.

16. Apparatus as in claim 14 wherein the means coupling the attenuatedenergy to the first detector includes the other polarization port of thefirst separator.

17. Apparatus as in claim 14 wherein the means coupling the attenuatedenergy to the first detector includes a second 90 polarization rotatorand means coupling the other polarization port of the first separator tothe second rotator.

18. Apparatus as in claim 14 wherein said material comprises a generallyplanar web and wherein the horns are so oriented that radiationtherebetween passes through the web at an appreciable angle from anormal to its plane.

19. Apparatus as in claim 1 wherein the source operates in the frequencyband from 20 GHZ to 25 GHz.

20. Apparatus as in claim 1 wherein the variable frequency sourcecomprises a backward wave oscillator.

21. Apparatus as in claim 1 wherein said material comprises a web andwherein the radiating means comprises a pair of horn antennas disposedon opposite sides of the web and mounted on a U-shaped member adapted tobe moved transversely across the width of the web.

22. Apparatus as in claim 1 wherein the construction of thelens-corrected horn is such as to provide a parallel beam having aplanar wavefront 23. Apparatus as in claim 1 wherein the means couplingthe attenuated energy to the first detector includes a secondlens-corrected horn.

24. Apparatus as in claim ll wherein the corrected horn includes aplano-convex converging lens.

25. Apparatus as in claim 1 wherein the corrected horn includes a doublerefraction converging lens.

26. Apparatus as in claim 1 wherein the corrected horn includes a singlerefraction converging lens.

27. Apparatus as in claim 1 wherein the corrected horn includes astepped approximation of a converging lens.

28. Apparatus as in claim 1 wherein the means coupling the attenuatedenergy to the first detector includes a second horn, the apparatusfurther including a band-pass filter and means coupling the detector tothe filter, the filter having a pass band sufficiently narrow to rejectthe fundamental component of modulation caused by mismatch reflectionsbetween the two horns.

1. Microwave moisture measuring apparatus including in combination avariable frequency source of microwave energy, control means forcyclically varying the frequency of the source, means including alens-corrected horn for radiating energy from the source through amoisture containing material, said material attenuating the energypassing therethrough as a function of its moisture content, a firstdetector, and means coupling both the source and the attenuated energyto the first detector.
 2. Apparatus as in claim 1 further including aband-pass filter, a second detector, and means including the filter forcoupling the first detector to the second detector.
 3. Apparatus as inclaim 1 wherein the control means cyclically varies the source frequencyat a certain repetition frequency, the apparatus further including anamplifier, a band-pass filter, a second detector, means including theamplifier and the band-pass filter for coupling the first detector tothe second detector, a low-pass filter, and means coupling the seconddetector to the low-pass filter.
 4. Apparatus as in claim 3 wherein thelow-pass filter has a cut-off frequency less than twice said repetitionfrequency.
 5. Apparatus as in claim 1 wherein the construction of thecontrol means is such that the source frequency cyclically varies inaccordance with a generally triangular waveform having equal positiveand negative slopes.
 6. Apparatus as in claim 1 wherein the constructionof the control means is such that the source frequency cyclically variesin accordance with a trapezoidal waveform having flat top and bottomportions of appreciable extent.
 7. Apparatus as in claim 1 wherein themeans coupling the attenuated energy to the first detector includes adelay line.
 8. Apparatus as in claim 1 wherein the means coupling thesource and the attenuated energy to the first detector includes adirectional coupler.
 9. Apparatus as in claim 1 wherein the radiatingmeans includes a microwave isolator.
 10. Apparatus as in claim 1 whereinthe means coupling the attenuated energy to the first detector includesa microwave isolator.
 11. Apparatus as in claim 1 wherein the radiatingmeans includes means for passing source energy twice through thematerial.
 12. Apparatus as in claim 1 wherein the radiating meansincludes a first transmitting horn antenna disposed on one side of thematerial, a first receiving horn antenna disposed on the other side ofthe material and oriented to receive energy from the first transmittinghorn, a second transmitting horn antenna disposed on said other side ofthe material, means coupling the first receiving horn to the secondtransmitting horn, and a second receiving horn antenna disposed on saidone side of the material and oriented to receive energy from the secondtransmitting horn.
 13. Apparatus as in claim 12 wherein the meanscoupling the first receiving horn to the second transmitting hornincludes a delay line.
 14. Apparatus as in claim 1 wherein the radiatingmeans includes a first and a second horn antenna disposed on oppositesides of the material and oriented to transmit and receive energy fromone another, a first and a second polarization separator each having acommon port and a pair of polarization ports, means coupling the sourceto one polarization port of the first separator, means coupling thecommon port of the first separator and the first horn, means couplingthe common port of the second separator and the second horn, and meansincluding a first 90* polarization rotator for coupling one polarizationport of the second separator and its other polarization port. 15.Apparatus as in claim 14 wherein the means coupling the two polarizationports of the second separator includes a delay line.
 16. Apparatus as inclaim 14 wherein the means coupling the attenuated energy to the firstdetector includes the other polarization port of the first separator.17. Apparatus as in claim 14 wherein the means coupling the attenuatedenergy to the first detector includes a second 90* polarization rotatorand means coupling the other polarization port of the first separator tothe second rotator.
 18. Apparatus as in claim 14 wherein said materialcomprises a generally planar web and wherein the horns are so orientedthat radiation therebetween passes through the web at an appreciableangle from a normal to its plane.
 19. Apparatus as in claim 1 whereinthe source operates in the frequency band from 20 GHz to 25 GHz. 20.Apparatus as in claim 1 wherein the variable frequency source comprisesa backward wave oscillator.
 21. Apparatus as in claim 1 wherein saidmaterial comprises a web and wherein the radiating means comprises apair of horn antennas disposed on opposite sides of the web and mountedon a U-shaped member adapted to be moved transversely across the widthof the web.
 22. Apparatus as in claim 1 wherein the construction of thelens-corrected horn is such as to provide a parallel beam having aplanar wavefront.
 23. Apparatus as in claim 1 wherein the means couplingthe attenuated energy to the first detector includes a secondlens-corrected horn.
 24. Apparatus as in claim 1 wherein the correctedhorn includes a plano-convex converging lens.
 25. Apparatus as in claim1 wherein the corrected horn includes a double refraction converginglens.
 26. Apparatus as in claim 1 wherein the corrected horn includes asingle refraction converging lens.
 27. Apparatus as in claim 1 whereinthe corrected horn includes a stepped approximation of a converginglens.
 28. Apparatus as in claim 1 wherein the means coupling theattenuated energy to the first detector includes a second horn, theapparatus further including a band-pass filter and means coupling thedetector to the filter, the filter having a pass band sufficientlynarrow to reject the fundamental component of modulation caused bymismatch reflections between the two horns.